An image processing apparatus, such as a scanner, has a light source that irradiates a sheet to be scanned and scans an image on the sheet based on the light reflected by the sheet. Specifically, the light reflected by the sheet is incident on a plurality of photoelectric transducers that is linearly arranged in a main-scanning direction, and an image data signal corresponding to intensity of the light is output from each of the photoelectric transducers.
Typically, the light source moves in a sub-scanning direction to irradiate a scanning region (scanning line) of the sheet. However, the light from the light source may not irradiate the scanning region in the same manner (e.g., same angle and same distance from the scanning region) as the light source moves in the sub-scanning direction. As a result, the obtained image data may not have the same brightness values in two different regions, even though the original sheet has the same brightness in the two regions.